Immunization to protect against communicable disease is one of the most successful and cost-effective practices of modern medicine. Smallpox has been completely eliminated by vaccination, and the incidence of many other dreaded diseases such as polio and diphtheria has been drastically reduced through immunization programs. However, vaccines, especially those based on the use of inactivated viruses, vary in effectiveness.
Thus, in order to enhance the immunogenic effects of vaccine compositions, adjuvants are frequently used. Such adjuvants are often aluminum compounds, in the form of hydroxides or phosphates. Through coupling with vaccine antigens, the so-called adsorbate, these aluminium compounds result in vaccines, the immunopotentiation action of which is explained by the resulting vaccine depots, from which the release of antigen for adsorption is retarded. However, the aggregation and settling tendencies of solid aluminium compounds, as well as the behaviour of their residue, is a disadvantage of the use of these compounds. In addition, further problems arise with vaccines adjuvanted with Al-salts due to a number of other reported limitations, e.g.    a) the antibody response is often of short duration,    b) they induce poor cell mediated immunity (CMI) and    c) they can support the production of IgE antibodies which may lead to hypersensitive reactions.
As an interesting alternative to the Al-salts, the saponins extracted from the bark of the tree Quillaja saponaria have been suggested (see, for example, WO98/155287, WO96/33739, U.S. Pat. No. 5,817,314, U.S. Pat. No. 5,977,081). In contrast to the Al-salts, these saponins are able to induce both antibody response and CMI. Therefore, they have been frequently used as adjuvants in veterinary vaccines. Unfortunately, most Quillaja-saponins (Qsaponins) have a strong haemolytical activity and cause undesirable local reactions (Tizard, I. Vaccination and Vaccines. In An Introduction to Veterinary Immunology 5th ed. Editor I. Tizard; Philadelphia, London, Tokyo, 1996, 265-284; and Horzinek, M. C., Schijns, V.E.C.J., Denis, M., Desmettre, P. and Babiuk, L. A. General Description of Vaccines. In Veterinary Vaccinology, ed. P. P. Pastoret, Blancou, J., Vannier, P. and Verschueren, C., Elsevier Science, Amsterdam, New York, Tokyo. 1997, 131-152), which limits their use in vaccines for large animals. Moreover, the ginsenoside-saponins from the Korean Red Ginseng Radix has been proposed to show adjuvant activity in its pure form. However, to purify a single ginsenoside is a costly process.
Thus, this far, saponins have not seemed to constitute ideal adjuvants either. Although the literature contains many reports describing new adjuvants, most vaccines recommended for human or veterinary use still contain aluminium salts (Al-salts) as the adjuvant, despite the above discussed disadvantages thereof. Conclusively, at present, there is still a need within this field for novel adjuvants, or at least for methods of reducing the amount used of the hitherto used Al-salts.